The many relationships of leaf-cutter ants

Are those little leaf scraps on the ground? Or is it a line of leaf cutter ants (Atta cephalotes)? Photo taken at La Selva Biological Station, December 2008 by Hannah Waters

Trying to capture the movement of a colony of leaf-cutter ants in a single photo is nearly impossible in my (amateur) experience. The queues of ants follow a worn-down trail in the ground that they themselves made with the impact of their little ant feet. There are ants moving in both directions, between the food source and their nest, but you rarely see them run into each other. (Though small ants, minima, will hitch rides on another ant’s leaf.) It’s an organized flow of ants and materials – but a photo can only capture the single frame, as if it’s just bits of leaf littering the ground. The sense of movement is lost.

But that’s only the the frustration if this is the aspect of ant symbiosis you’re trying to capture: the leaf cutter ants collecting foliage to feed to their cultivated fungus. What if it’s a different symbiotic relationship you’re interested in?

BOOM! Image by mass spectrometer to measure the amount of an antibiotic, valinomycin, on a leaf cutter ant, Acromyrmex. by Schoenian et al. 2011.

Leaf cutter ants, of the genera Atta and Acromyrmex, live in huge colonies and can form nests more than 30 meters in diameter. They’re most noted for their agricultural behavior: they harvest leaves (and can defoliate entire trees in the process) which they bring back to their nests to feed their mutualistic fungus (of the family Lepiotaceae), which acts as a food source for the ants in exchange for the food.

Imagine being in that nest, inside a cavern full of warm, wet fungus. Most organisms would be looking forward to gorging on this food source, or to growing there themselves. The ants meticulously clean their fungus gardens and even secrete an antimicrobial acid to keep themselves from bringing in outside visitors, but is that enough to fend off the various microbial and fungal intruders?

In 1999, Cameron Currie (whose ant colony has a twitter account) identified a microbe (Streptomyces) living on the ants’ bodies that produces an antibiotic chemical. He found this same microbe living not just on one, but many leaf-cutter ant species, including two genera of fungus-growing ants that evolved earlier than the leaf-cutters Atta and Acromyrmex we know and love. This microbe produced an antibiotic that was very potent against a common garden-invading fungus, Escovopsis. So it seemed that this was a 3 way mutualism: the ant, the fungus it feeds on, and the microbe that helps protect the garden.

Is anything in biology ever so simple? The hunt began to find more bacterial antibiotic-producing symbionts. Scientists have found many more symbionts and garden parasites, revealing the complexity of this ecosystem. But the microbial symbionts aren’t always masked superheroes protecting the fungal garden: their antibiotics can also do harm to the farmed fungus itself. Additionally, if there are many species of microbes around, an evolutionary arms race can emerge in which the microbes try to outcompete one another, creating stronger and stronger antibiotics which could be harmful.

Instead of simply listing off the species of microbes they discover on ants, Dieter Spiteller and his lab have been working to identify the antibiotics they produce. This approach allows them to more easily identify what antibiotic types work on what pathogens and parasites and see what combinations of antibiotics show up together. After all, the ants don’t care much about what species of microbe lives on them: only the benefit of the antibiotics.

Many of these microbial symbionts are not well-studied, if at all, so figuring out what type of antibiotic they are producing is a bit of a trick. In their recent paper, the researchers used the evolutionary relationships of the symbionts – constructed using 16S rDNA sequences, which are commonly used to measure the “closeness” of two species – to find their closest relative that had already had its antibiotic identified by scientists. This allowed them to narrow down their screen to look for the antibiotics that were most likely being produced by each bacterial species instead of having to screen for ALL of them.

They identified many types of antibiotics that fell into three main categories: antimycins, valinomycins, and actinomycins, the latter so toxic that they aren’t used as antibiotics in humans due to damage to surrounding cells. Then they tested the effectiveness of each antibiotic against a variety of pathogens and parasites that could endanger the nest: fungal pathogens of the gardens and insects, the common soil bateria Bactillus subtilis, microbial isolates taken from the ants and garden, and even the mutualistic fungus itself, Leucoagaricus gongylophorus.

The antibiotics varied in their strength and specificity. Unsurprisingly, the most potent was the combination of the three – thus biochemical and microbial diversity is quite important to the success of these ants. However, the antibiotics can have detrimental effects on other microbes living on the same ant (in particular actinomycin). But this competition encourages the selection of stronger antibiotics in other species, which is beneficial to the ant and thus the symbiosis overall.

And that brings us back to that second image above: a visualization of the valinomycin distribution on a single ant. As you can see, it’s pretty widespread – and is found more regularly in a high concentration than in the fungus gardens. This makes logical sense: an ant would want to kill as many pathogens as possible BEFORE it brought the leaf into the nest. But it also keeps the antibiotics from killing off too much of the ants’ fungus food.

To sum up: I just wrote a post about antibiotics produced by bacteria, which are competing for space on the back of an ant. This ant climbs trees, cuts down leaf pieces, and carries them back to its nest to feed to a fungus which it then eats. The antibiotics produced by the bacteria on its back keep pathogens at bay – protecting the fungus garden, thus the ant, and thus the bacteria. Phew!